This invention relates to optical projection lamps and more particularly to high intensity discharge (HID) electric lamps for optical projectors which lamps are presently generally constructed with quartz envelopes.
At the present time lamps (bulbs) for optical projectors are generally of the high intensity discharge (HID) type in which an arc is formed between two electrodes, the electrodes being positioned at opposite ends of a tubular envelope with a gap between them. The light from the lamp is reflected from a reflector and focused on an image gate, for example, an LCD (Liquid Crystal Display) plate, a slide projector film gate or motion picture film gate.
HID lamps presently have light transmissive lamp envelopes with quartz or ceramic (polycrystalline). Many lamp patent claims are based on benefits arising from specific forms of these materials. For example, in U.S. Pat. No. 4,501,993, relating to an electrodeless lamp bulb for producing deep ultraviolet (UV) xe2x80x9csynthetic quartz which is substantially water freexe2x80x9d is claimed as an advantage over xe2x80x9ccommercial quartz.xe2x80x9d In the article xe2x80x9cMetal Halide Lamps with ceramic Envelopes: A Breakthrough in Color Control,xe2x80x9d Journal of the Illuminating Engineering Society, Winter, 1997, the advantages of translucent polycrystalline alumina ceramic envelopes over quartz envelopes are highlighted.
However, the light transmissive envelope technologies in present use have limitations which affect the ability of such lamps to provide long life, flicker-free operation, color stability and high efficacy.
The limitations quartz envelopes impose on HID lamp performance include the following:
1. The envelope structures are physically delicate and subject to breakage in handling;
2. Devitrification by water, and many different chemicals such as hydrogen and chlorine, limit the light output and the lifetime of electric lamps.
3. Sodium, neon and hydrogen diffuse out of the bulb and so they cannot be used for fills.
4. Pressure is limited by the tensile strength of 7000 lb/in{circumflex over ( )}2 at room temperature.
5. Large temperature gradients occur across the bulb wall, limiting the heat transfer capability of the wall to about 20 watts/cm{circumflex over ( )}2.
Despite these limitations, quartz envelopes are generally used because ceramic (polycrystalline) envelopes present greater limitations. The limitations imposed by ceramic (polycrystalline) walls include:
1. The ceramic is a translucent material which is unsuitable for optical systems.
2. The ceramic envelope is brittle.
3. Such ceramic envelopes have a relatively low tensile strength of less than 25,000 lb/in{circumflex over ( )}2.
Lamp systems of quartz and ceramic (polycrystalline) envelopes have been in commercial use for many years and in most application areas, lamp performance has been optimized to the physical limits of these materials.
In some LCD (Liquid Crystal Display) projector electrode HID lamp applications it is desirable to have short (1-2 mm) arc gaps and 1-2 mm diameter for the light emitting volume. Such applications also need light emitting volumes that produce efficacy of 60 lumens/watt, or more, with good color stability, flicker-free operation and lifetimes of more than 2000 hours.
An example of a system maximized to the physical properties of quartz is described in Matthews et al U.S. Pat. No. 5,239,230. This patent describes the maximum performance capabilities of a short arc HID discharge lamp with a Mercury, Bromine, Xenon fill. The inner bulb diameter is limited to dimensions greater than 3.8 mm for power levels of 70 to 150 watts. Limitations are due to hoop stress limitations and temperature limitations, on the inner wall of the quartz tube, which result in melting of the inner bulb surface causing failure in less than 100 hours.
Another example of a system maximized to the physical properties of quartz is described in Fischer U.S. Pat. No. 5,497,049. This patent describes the maximum performance capabilities of a specific HID high-pressure mercury (over 200 bar) discharge quartz envelope design for LCD projectors having tungsten electrodes. Such a system suffers from premature failure due to devitrification and blackening of the inner bulb surface in the arc region and in the tip-off regions. Such lamps utilize bromine as an enhancer of efficacy but cannot use chlorine because of reactions with the envelope and cathode materials. The authors find the inner diameter of the bulb has to be greater than 3.8 mm for lamps in the 70-150 watt range to avoid premature failure due to the physical properties of the quartz.
Electrodeless lamps filled with sulfur and selenium have superior luminance properties. See, for example, U.S. Pat. No. 5,404,076 dated Apr. 4, 1995, and U.S. Pat. No. 5,606,220 dated Feb. 25, 1997. However, the envelopes are made of quartz, which has an operating temperature limitation of 900xc2x0 C. For example, the xe2x80x9cLight Drive 1000xe2x80x9d lamps developed by/Fusion Lighting Inc. utilize quartz envelopes and require constant rotation at high rpm to avoid development of hot spots that create temperatures of over 900xc2x0 C. If the rotation stops, the bulb blows up in about 3 seconds.
Lamp systems composed of ceramic (polycrystalline) material are translucent and are thus not usable for many optical systems applications. They are also brittle and have relatively low tensile strength. They do have advantageous features for lamp envelope applications in that they are chemically inert and impervious to elements like sodium, hydrogen, neon, chlorine, etc. For example, color stability and efficacy of over 90 lumens/watt of HID lamps with ceramic (polycrystalline) envelopes are described by Carleton et al in xe2x80x9cMetal Halide Lamps With Ceramic Envelopes: A Breakthrough in Color Controlxe2x80x9d, published in the Journal of the Illuminating Engineering Society, Winter, 1997.
Flash lamps, without continuous arcs, have been fabricated from single crystal (SC) sapphire by ILC Corporation of California and by Xenon Corporation of Massachusetts. SC sapphire is alumina (aluminum oxide) formed as a single crystal. These lamps have been demonstrated to have superior lifetime and color maintenance over quartz. The end seals of these commercial lamps utilize metal brazing materials and kovar components, which are unsuitable for HID lamp applications.
There are examples in the literature of seals to ceramic (polycrystalline alumina) tubing which have proven adequate for xe2x80x9cdouble wallxe2x80x9d containment vessels which have an outside envelope of quartz. For example, Juengst et al U.S. Pat. No. 5,424,608, Pabst et al U.S. Pat. No. 5,075,587, and Bastian U.S. Pat. No. 5,455,480 describe such sealing arrangements using a variety of glass sealing materials optimized for sealing to polycrystalline materials.
U.S. Pat. No. 5,702,654 relates to manufacture of single crystal sapphire for windows and domes. U.S. Pat. No. 4,018,374 relates to a sapphire-glass seal. U.S. Pat. No. 5,451,553 relates to thermal conversion of polycrystalline alumina to sapphire by heating to above 1100xc2x0 C. and below 2050xc2x0 C., and U.S. Pat. No. 3,608,050 relates to growing single crystal sapphire from a melt of alumina. The only mention we found in the patent literature of clear sapphire in a lamp is in a radio luminescent lamp application described in U.S. Pat. No. 4,855,879 in which clear sapphire planar window material is mentioned. The only mention we found in the technical literature is a diagnostic sodium discharge lamp described by S. A. R. Rigten, Gen. Elec. Co.J., Vol.32, p.37, 1965, in which a transparent sapphire tube is used for diagnostic purposes.
One of the difficulties in utilizing single crystal (SC) sapphire in commercial lamp construction is the difficulty in growing the cylindrical crystals with suitable concentricity and a crystalline structure free of defects. The above-mentioned patents and articles are incorporated by reference.
This invention significantly improves the efficacy, lifetime, and color stability of high intensity discharge (HID) lamps, especially projector lamps. It uses single crystal (SC) sapphire bulb envelopes which have physical properties superior to those of quartz and ceramic (polycrystalline) bulb envelopes. Its principal object is to provide a novel high intensity discharge (HID) lamp with a light transparent envelope of single crystal (SC) sapphire. The SC-sapphire HID lamp can be smaller, operate at higher power for equal size and be brighter with higher plasma luminance than quartz lamps with similar dimensions and fills. SC-sapphire HID lamps can also last four to five times longer with superior lumen maintenance. Such lamps may be easier to manufacture with superior manufacturing tolerances and at the same or lower cost as fused quartz envelopes, or polycrystalline alumina envelopes. These sapphire lamps use metal to ceramic seals that can tolerate temperatures up to 1300xc2x0 C. as compared to fused quartz to metal seals that are limited to temperatures of about 250xc2x0 C. The SC-sapphire HID lamp is preferably powered through two end electrodes or less preferably a combination of electrodes and microwave sources.
An object of the invention is to provide a novel sulfur or selenium-filled lamp with a light transparent envelope of single crystal (SC) sapphire.
Another object of the invention is to provide a novel method of sealing lamps having SC-sapphire envelopes in such a way that the lamps can contain light emitting gaseous substances with pressures as high as 600 atmospheres.
Another object of this invention is to provide a novel method of assembly of lamps with SC-sapphire envelopes in such a way that the manufacturing costs are low.
This invention will make possible a wide range of new lamps based on SC-sapphire envelopes with application in optical projectors. The lamp may also be used in automobile headlamps and home and general lighting applications.